Teach --recursive to submodule sync

Teach --recursive to submodule sync

The submodule sync command was somehow left out when
--recursive was added to the other submodule commands.

Teach sync to handle the --recursive switch by recursing
when we're in a submodule we are sync'ing.

Change the report during sync to show submodule-path
instead of submodule-name to be consistent with the other
submodule commands and to help recursed paths make sense.

Signed-off-by: Phil Hord <hordp@cisco.com>
Acked-By: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Jeff King <peff@peff.net>